The only random drop mounts I don't have from old content are the roic only ones (raid and ZG), Experiment 12-b and Al'ar. I was quite frankly surprised at how quick I got the Fiery Warhorse (7 or 8 runs) and the Onyxian Drake (1 Run). I just finished up the 5 man drakes from cata over the last month, and got the Quiraji from Archaeology a few days ago. I actually hit 200 mounts yesterday, on my birthday no less. I fully admit I am lucky on some mounts, but some I've had to grind for. Rivendare took me 213 kills on my DK to get back in Cata, besides the 100+ kills on other toons from before.

I got Aeonaxx... on the Pandaria Beta, so unfortunately it didn't count.

I finally finally got Midnight last week. Was running it 19 times a week, got it on my 208th run since returning to the game (end of june), I easily did more than 1000 kills, likely closer to 2000. I used to run Kara up to 5 times a week during TBC. Solo'd through WotLK and Cata too.

Never really cared for it but once I helped a friend through Karazhan because he never got the chance to do it himself in the early days. The mount dropped but as the nice guy I am, I "gave" it to him. Never seen it since and he doesn't play anylonger not that I do either.

I got it the 3rd or 4th time there. I don't really remember because I wasn't really farming for it. Also having the halloween mount with a similar look but also being able to fly, the kara horse doesn't get much action. The most amount of time I have ever spent farming something was either the crimson whelpling or the emerald whelpling back in vanilla. I don't remember how much time I spent, but it was a hell of a lot of hours. For a mount, probably the raven lord took the longest for me.

I got firey warhorse first try, zulian tiger first try, anzu 3rd try (and 5th try), strat mount 5th try, alar took me quite some time, vitreous took me almost 100 kills, ony took me a while and just got it recently, im well over 115 kills for my blue proto and still havnt gotten it, and i've killed houlon almost 100 times now and no mount.

im either very lucky or extremely unlucky.. its weird..

edit: and with october right around the corner i just remembered. no HH mount after 3 years of 2-3 characters running it EVERY day EVERY year. friend got 5 of them one year, and i see atleast 4-5 people each year open their bag and get one before leaving the dungeon..

I saw it(Atumen) drop two times. First during the time when Mirkblood Event was live. And second during Cataclysm. My ExGf was always so frustrated because of her really, really bad internet connection so i went with her these old instances and brought her epic items from the AH. You know, happy girlfriend = no reason to shove her down the stairs
Long story short, it dropped, i pass, sho won, was happy, no shoving, now i´m bitting my ass off, still farming this shit.
Thanks jesus that she wasnt around when Alar dropped.

I never farm those from hearing the horror stories, but I'd probably be one of those guys that get it in a few tries and feel bad for the people that spent years on it. Or, I'd get obsessed like many others and spend 2-3 years attempting.. fuck that lol.